Cavity ring-down spectroscopy (CRDS) is an analytical technique where optical radiation emitted from a passive optical cavity is measured as a function of time. The decay rate of this emitted radiation is related to the loss in the cavity (lower loss leads to slower decay). Typically, an exponential decay is fitted to the measured radiation intensity to determine the ring-down time. Absorption caused by an analyte in the cavity affects the ring-down time, so measuring the ring-down time amounts to a highly sensitive form of absorption spectroscopy. The resulting CRDS instruments are widely applicable to various analysis applications, especially in cases requiring ultra-high sensitivity (e.g., part per billion level).
Ideally, only a single cavity mode is relevant during a ring-down measurement, with all other modes having negligible amplitude. The reason for this is that intracavity loss will tend to have a different effect on the decay rates of each of the cavity modes, so a clean single-exponential decay can only be obtained for single-mode excitation.
The cavity in a CRDS instrument can be either a standing wave cavity or a traveling wave cavity. A typical example of a standing wave cavity is a two-mirror cavity where a round trip of the cavity mode entails propagating on the path between the two mirrors once in each direction. A typical example of a traveling wave cavity is a three mirror ring cavity, where a round trip of the cavity mode entails propagating on a path around the ring cavity (e.g., in a clockwise or counter clockwise direction).
In CRDS using a traveling wave cavity, one typically distinguishes between the forward mode, which is driven by the optical source of the CRDS instrument, and the backward mode which is at the same frequency as the forward mode, but propagates in the opposite direction. For example, if the forward mode propagates clockwise around a ring cavity, the corresponding backward mode propagates counter clockwise, and vice versa.
Ideally, the amplitude of the backward mode would be zero, so this mode is usually neglected in conventional accounts of CRDS operation. However, there is some consideration of the backward mode in the art. In U.S. Pat. No. 7,646,485, two ideas are considered. The first idea relates to performing more complicated curve fitting than a simple exponential to mitigate the effect of backward mode artifacts on CRDS results. The second idea relates to measuring excitation of the backward mode by a source aligned to nominally excite only the forward mode to provide an indication of the quality of the cavity alignment. Adjustment of the cavity alignment during assembly to minimize the measured excitation of the backward mode can be helpful for improving the cavity alignment of the finished instrument.